1. Field of the Invention
This invention relates to an improved means for delivering a flow of medication into an animal or a human subject. Although the device described herein is clearly capable of being fully implanted in a living body, it is also readily capable of being used as an extracorporeal means for medication infusion.
2. Description of the Prior Art
In recent years, many advantages have been found in treatments in which medication is infused directly into a patient's body by means of a portable or implantable pumping system. An optimum system for many patients is an implantable medication pumping system that is simple, reliable, small, lightweight, and provides a precise medication infusion rate irrespective of ambient pressure, body temperature or the extent to which the reservior has been emptied.
One of the earliest implantable pumps is described by P. Blackshear et al. in U.S. Pat. No. 3,731,681. However, the Blackshear et al. device has several disadvantages:
a. It uses the movement of a bellows to dispense the medication. Any bellows will have a spring constant force F=-kd, where k is the spring constant of the bellows and d is its displacement from equilibrium. Since drug flow can be directly related to net force of the bellows and this force will vary with the displacement, the medication flow rate will be dependent on bellows displacement.
b. Blackshear's device utilizes a "propellant chamber" which is separated from the medication chamber by a metal plate. The driving force derived from this propellant chamber is independent of ambient atmospheric pressure. The medication flow rate, on the other hand, is related to the difference in pressure between the propellant chamber and ambient pressure. Thus, as ambient pressure changes, the flow rate will change. This is likely to occur due to barometric pressure variations, flying in a jet plane, being at a different altitude, going up in an elevator, diving to the bottom of a swimming pool, etc.
c. The preferred embodiment of the Blackshear et al. device uses a fluorocarbon as the propellant. Fluorocarbons have a strong variatio in vapor pressure as the ambient temperature changes. Thus, if the patient's body temperature increases or decreases, the propellant pressure and thus the medication flow rate will change.
d. The flow rate of the Blackshear et al. pump depends on the resistance of its flow restrictor, which in turn depends on the viscosity of the medication, which is temperature dependent and which varies somewhat with the types and concentration of the medication.
e. To lessen the effects of the bellows spring force, ambient pressure changes and body temperature changes, the Blackshear et al. device, in practice, uses a relatively high pressure propellant. This is cause for concern because a leak of medication is potentially lethal.
f. The high pressure used with the Blackshear et al. device makes refilling the pump reservoir a more difficult task.
g. The Blackshear et al. pump does not allow the patient to alter the flow rate of medication; a feature often desirable for the treatment of medical problems.
In U.S. Pat. No. 4,146,029, Ellinwood describes a number of implanted medication systems. All the devices he describes require power sources and electronic circuitry which make them complex, expensive, and shorten operating life.
R. E. Fischell in U.S. Pat. No. 4,373,527 describes a complex implantable infusion pump which is not only expensive and less reliable, but has a shorter operating life when compared to a simple, fully mechanical pumping system. In addition, the Fischell device also exhibits a variation in flow rate with changing ambient pressure, and to a lesser extent with body temperature and the extent of the fullness of its reservoir.
Recently, the implantable medication pump has shown promise in the treatment of intractable pain, i.e., chronic or remittent pain which is refractory to conventional therapy. Traditionally, a number of different methods have been utilized to treat such pain. These include palliative or "curative" therapy (i.e., surgery, radiation therapy, or chemotherapy), systemically administered narcotics, transcutaneous electrical stimulation, nerve blocks, rhizotomy, radiofrequency induced lesions, epidural or dorsal column electrical stimulation, and central nervous system neurosurgical intervention (e.g. cordotomy, thalamotomy, acupuncture, and hypnosis).
The large number of modes of therapy is generally indicative of the ineffectiveness of these treatments. The three largest groups of patients requiring the treatment of intractable pain are:
Terminal Cancer Patients: Typically with metastases to bone, visceral organs or compression of neural structures; incidence is approximately 200,000 new patients each year. PA1 Chronic Lower Back Pain: Failure after one or more surgical interventions; approximately 200,000 current U.S. cases. PA1 Other Patients: Post-herpetic neuralgia, phantom limb pain, trigeminal neuralgia, etc.
Recently a number of medical centers have been using a new method for the treatment of intractable pain involving the epidural or intrathecal infusion of morphine. As this treatment has gained acceptance, a number of devices have been used. Unfortunately, each has a number of serious disadvantages.
The Broviac catheter, which allows percutaneous infusion, requires the catheter to actually extend through the patient's skin. While this allows injections to be made easily into the cerebro-spinal fluid, the risk of infection and patient discomfort preclude it from being a long term solution for intractable pain.
A large plastic reservoir with associated valves was described by Poletti et al. ("Cancer Pain Relieved by Long-Term Epidural Morphine with Permanent Indwelling Systems for Self Administration," Journal of Neurological Surgery, October 1981). This system utilizes a plastic bag implanted subcutaneously containing a volume of 400 ml of morphine. This approach is potentially dangerous should a leak occur, and requires surgery to replace the bag of morphine as it is not refillable. In addition, the valve system used allows the patient to infuse excessive amounts of morphine with no limit. On the whole, this system is potentially dangerous and not a practicable long-term solution.